A ballast (at least as the term is used herein) is something which is employed to limit the level of a current through a gaseous-discharge lamp. For example, an inductor functions as a ballast when the inductor is connected in series with a fluorescent lamp across a 120 volt, 60 Hz, AC power line. Although satisfactory for many applications, this combination is less than ideal. For one thing, such a combination presents a less than ideal power factor to the AC power line. In addition, at 60 Hz, inductors (of any reasonable size) dissipate a relatively large amount of power generating a relatively large amount of heat. Further, fluorescent lamps operate more efficiently when driven from a high-frequency source of AC power, such as, for example, the AC power source which is disclosed in the U.S. Pat. No. 4,415,839 of Ronald A. Lesea, entitled "Electronic Ballast For Gaseous Discharge Lamps."
Portions of the above-mentioned ballast are illustrated (herein) in FIG. 1 of the drawing generally designated by the number 10. Illustrated with ballast 10 is a (load that employs at least one) gaseous discharge lamp, which is designated 12. Ballast 10 is shown to employ a power-factor-correcting network 20; a DC power supply 22; a pair of switches (transistors), which are respectively designated 24 and 26; a current-limiting network 28; and a pulse generator 30. DC power supply 22 is of a voltage-doubler type which develops two DC potentials with respect to a "reference" potential level that is developed on a line 42. DC power supply 22 develops on a "DC power-supply line" 44 a (twice peak) potential level and on a lamp "return" line 46 a potential level one half the line 44 potential level. To improve the power factor DC power supply 22 presents to an AC power line (by restricting the amount of power the DC power supply can obtain from the AC power line during peaks of the line cycle), the DC power supply is connected in series with power-factor-correcting network 20 across a 120 volt, 60 Hz, AC power line, which is represented by a "neutral" line 48 and a "hot" line 50. Switches (transistors) 24 and 26 are connected in a totem-pole configuration in which the channels of the transistors are connected in series between DC power-supply line 44 and reference line 42. In other words, the channel of switch (transistor) 24 is configured with one end of the channel connected to line 44 and with the other end of the channel connected to a ("high-frequency AC power-source") line 56; and, the channel of switch (transistor) 26 is configured with one end of the channel connected to line 56 and with the other end of the channel coupled (by a resistor, not shown) to line 42. The gates of switches (transistors) 24 and 26 are each coupled by a respective one of two lines, designated 62 and 64, to pulse generator 30. (In one embodiment) pulse generator 30 is configured to drive the switches (transistors), in turn, so as to develop on line 56, a source of high-frequency AC power, the waveform of which approximates a square wave. Lamp (load) 12 is coupled by current-limiting network 28 between high-frequency AC power-source line 56 and return line 46. Specifically, current-limiting network 28 is connected between line 56 and a line 68; and, lamp (load) 12 is connected between lines 68 and 46. (As will become apparent shortly, in some embodiments, current-limiting network 28 is also connected to return line 46.)
For purposes of discussion, for a moment, assume that lamp 12 includes but a single fluorescent lamp of the (four-foot long) type which is commonly designated F40T12 (fluorescent, 40-watt, tubular, twelve-eights-inch diameter). Normally, an F40T12 lamp requires a potential level (voltage drop) of approximately 200 volts for ignition and operates with a current level of approximately 0.4 amperes (at 60 Hz), developing a voltage drop of approximately 100 volts. Also, for a moment, assume, to ignite the lamp, that ballast 10 develops on line 56 a source of high-frequency AC power having a peak potential level (which, for a square wave, is the same as the RMS potential level) of 200 volts (somewhat more than the 140 volts actually developed). To limit the level of the current through lamp 12, first, assume, that current-limiting network 28 of ballast 12 (shown in FIG. 1) includes but a simple series resistor 200, as is illustrated in (prior art) FIG. 2A. Assume, to limit the level of the current through lamp 12 to 0.4 amperes, that resistor 200 has a resistance of 250 ohms, dropping 100 volts of the 200 volts developed on high-frequency AC power-source line 56. Then, as a consequence, resistor 200 would dissipate, as heat, 40 watts of power. Further, as a consequence, ballast 10 (shown in FIG. 1) would be required to provide 80 watts of power. Obviously, such a ballast (network) would not be very efficient.
Next, to limit the level of the current through lamp 12, assume that current-limiting network 28 of ballast 10 (shown in FIG. 1) includes but a simple series inductor 220, as is illustrated in (prior art) FIG. 2B. Assume, to limit the level of the current through lamp 12 to 0.4 amperes, that inductor 220 has a reactance of 380 ohms (at the square-wave fundamental frequency). (Since the voltages are in phase quadrature, the level of the voltage drop developed across inductor 220 is equal to the square root of 182 volts (the Fourier-adjusted level of the voltage (200 volts assumed) developed on high-frequency AC power-source line 56) squared minus 100 volts (the level of the voltage drop developed across lamp 12) squared, which equals approximately 152 volts. The reactance of inductor 220 is equal to 152 volts (the level of the voltage drop developed across the inductor) divided by 0.4 amperes (the level of the current), which equals approximately 380 ohms.) In this case, ballast 10 would be required to provide 80 VA into a load which is highly inductive. (Of course, inductor 220 stores, rather than dissipating as heat, in this case, approximately, 61 VA (152 volts times 0.4 amperes.) Another disadvantage of using but a simple series inductor (220) for current-limiting network 28 is that the inductor must be relatively large in order to handle the 61 VA. Yet another disadvantage is that the switches (transistors) must have the "generating capacity" to "generate" the total VA. The relatively high current level (0.4 amperes) through the effective (drain-to-source) "on resistance" of the switches (transistors) 24 and 26 results in a relative high power dissipation level in the transistors. As a consequence, ballast 10 would not only be relatively inefficient, but would require relatively efficient (large and expensive) heat sinks. (The effective transistor on resistance may be reduced by using (larger and) more expensive transistors.) (In the U.S. Pat. No. 4,684,850 of C. Stevens, a current-limiting network (ballast) including but a simple series inductor is shown, in FIGS. 4 and 5, designated 53. A current-limiting network (ballast) including but simple series capacitors (one for each of several lamps) is shown in the U.S. Pat. No. 4,613,796 of D. Bay, designated 57.)
Illustrated in (prior art) FIG. 2C, is a current-limiting network that includes an inductor 240 connected between high-frequency AC power-source line 56 and line 68 and a capacitor 242 connected between line 68 and return line 46. Such a current-limiting network, which is referred to herein as a "low-pass" "L-C" or "L-section" network, is useful in that it may be used to provide an impedance transformation, providing a relatively high impedance to lamp 12 while providing a relatively low impedance at, or near, a zero degree phase angle, to switches (transistors) 24 and 26, reducing the level of the current through the transistors. (Such a "low-pass" "L-C" or "L-section" current-limiting network (ballast) is shown, designated 65 and 63 in FIG. 3, of the U.S. Pat. No. 4,370,600 of Z. Zansky and, designated, "L" and "C" in FIG. 1 of the U.S. Pat. No. 4,346,332 of J. Walden. Further, such a network is used in conjunction with a step-up auto-transformer in the U.S. Pat. Nos. 4,532,456 and 4,525,649 of W. Knoll et al. The '456 network drives a single lamp; and, the '649 network drives multiple lamps.) Further, such a current-limiting network is useful in that it may be used to provide a "resonant rise" for starting lamp 12. (For example, if inductor 240 had a reactance of j200 ohms and if capacitor 242 had a reactance of -j300 ohms (both at the square-wave fundamental frequency), the delta (difference) would be 100 ohms and there would be a net three times voltage rise across capacitor 242.) (Protection for the components of an L-section current-limiting network (ballast) at resonance is the subject of a number of the series of United States patents of O. Nilssen, including U.S. Pat. No. 4,461,980.)
Finally, illustrated in (prior art) FIG. 2D, is a current-limiting network that includes an inductor 260 connected between high-frequency AC power-source line 56 and a node 262, a capacitor 264 connected between node 262 and return line 46, and another inductor 266 connected between node 262 and line 68. Such a current-limiting network, which is referred to herein as a "low-pass" "L-C-L" or "T-section" network, is useful in that it provides an extra degree of "design freedom" not provided by the current-limiting network shown in FIG. 2C. (Such a "low-pass" "L-C-L" or "T-section" current-limiting network (ballast) is shown in the U.S. Pat. Nos. 4,358,712 of L. Filgas, Jr. et al; 4,544,863 of K. Hashimoto; 4,277,728 of C. Stevens; and 4,641,061 of R. Munson. The components (L, C, and L, respectively) of the current-limiting networks are designated 10, 12, and 18 in FIG. 1 of the L. Filgas, Jr. et al patent; "L," "C2," and "C1" in FIG. 4 of the K. Hashimoto patent; 47, 46, and 45 in FIG. 4A of the C. Stevens patent; and L1, C1, and C4 (L2, C2, and C5 or L3, C3, and C6) in the R. Munson patent.
It is important to note that the networks illustrated in FIGS. 2B-D provide attenuation of the level of the harmonics of the square-wave frequency. As a consequence, the waveform of the "high-frequency AC power" actually driving lamp(s) 12 is much closer to a sinusoidal wave rather than a square wave.
The above mentioned ballasts are disadvantageous in that they provide little isolation from the AC power line. As a consequence, the above mentioned ballasts may pose a safety hazard (danger of electrocution) to all who come in contact there with.